1. Field of the Invention
The present invention relates to a radiation image information reader, and more particularly to a radiation image information reader for photoelectrically reading photostimulated luminescent light emitted from a stimulable phosphor sheet.
2. Description of the Related Art
A stimulable phosphor stores part of radiation energy when exposed to radiation, and emits photostimulated luminescent light according to the stored energy when exposed to excitation light such as visible light, laser light, etc. This stimulable phosphor is stacked on a support body and utilized in a radiation image recording-reproducing system, which has extensively been put to practical use. In the radiation image recording-reproducing system, the radiation image information of a subject, such as a human body, etc., is temporarily recorded on the stimulable phosphor sheet. The stimulable phosphor sheet emits photostimulated luminescent light when scanned with excitation light such as laser light, etc. The photostimulated luminescent light is detected photoelectrically by image read means, and an image signal representing the radiation image information is obtained. After this image signal has been read, the stimulable phosphor sheet is irradiated with erasing light and emits the radiation energy remaining therein.
The image signal obtained by the aforementioned radiation image recording-reproducing system is subjected to image processing, such as a gradation process, a frequency process, etc., suitable for image observation and reading. After these processes, the processed image signal is recorded on film as a visible image for diagnosis, or displayed on a high-definition CRT display, so that it can be used for diagnosis. On the other to hand, if the aforementioned stimulable phosphor sheet is irradiated with erasing light to remove residual radiation energy, the sheet can be repeatedly used because it can store and record radiation image information again.
It is disclosed (in Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, 1(1989)-101540, etc.) that the radiation image information reader in the aforementioned radiation image recording-reproducing system employs a line light source and a line sensor in order to shorten the time needed to read photostimulated luminescent light, make the reader compact, and reduce costs. The line light source is used as an excitation light source for irradiating line excitation light to a phosphor sheet. The line sensor is used as photoelectric read means, which includes a large number of photoelectric conversion elements arrayed along the length direction of a line portion on the sheet irradiated with the excitation light by the line light source. Furthermore, the radiation image information reader is equipped with scan means for relatively moving the line light source and the line sensor with respect to the phosphor sheet in a direction substantially perpendicular to the length direction of the aforementioned light-irradiated line portion.
However, since photostimulated luminescent light spreads in all directions from a spot on the stimulable phosphor sheet irradiated with excitation light, the photostimulated luminescent light is detected not only by a photoelectric conversion element corresponding to the irradiated spot but by photoelectric conversion elements other than the corresponding photoelectric conversion element. Therefore, in the case where excitation light is irradiated to the stimulable phosphor sheet by the aforementioned line light source, and photostimulated luminescent light is detected by the line sensor, the beams of photostimulated luminescent light are mixed at the photoelectric conversion element of the line sensor and cause crosstalk. As a result, there is a problem that the sharpness of a radiation image obtained will be reduced. In this case, to avoid crosstalk, photostimulated luminescent light can be narrowed down so that it is detected only by a corresponding photoelectric conversion element. However, this causes another problem that photostimulated luminescent light cannot be efficiently collected.
The present invention has been made in view of the circumstances mentioned above. Accordingly, it is an object of the present invention to provide a radiation image information reader that is capable of shortening the time needed to read photostimulated luminescent light, as in the aforementioned radiation image information recording-reproducing system where line excitation light is irradiated onto a phosphor sheet by the line sensor. Another object of the invention is to provide a radiation image information reader which is capable of suppressing crosstalk.
To achieve the objects of the present invention mentioned above, there is provided a radiation image information reader for reading radiation image information from a stimulable phosphor sheet and obtaining an image signal which represents the radiation image information, the radiation image information reader comprising horizontal scan means and read means. The horizontal scan means is used for horizontally scanning a plurality of spot-sized excitation light beams simultaneously onto the phosphor sheet at predetermined intervals on a horizontal scanning line. The read means is used for obtaining the image signal which represents the radiation image information by photoelectrically detecting photostimulated luminescent light beams, emitted from portions of the sheet irradiated with the excitation light beams and/or from portions on a bottom surface of the sheet which correspond to the irradiated portions, by horizontal scanning of the excitation light beams.
An excitation light source that is employed in the horizontal scan means can use a light-emitting element array, a laser array, a combination of a plurality of laser light beams and deflection means for reflecting and deflecting these laser light beams, etc. The plurality of laser light beams may be emitted from a plurality of lasers, or may be obtained by emitting a single laser light beam from a single laser and then dividing the single laser light beam into a plurality of laser light beams with a beam splitter. The suitable linewidth of the excitation light beam on the sheet is 10 to 4000 xcexcm. A suitable number of excitation light beams is from 2 to 100.
The predetermined interval is an interval such that a photostimulated luminescent light beam emitted by the irradiation of one excitation light beam onto the sheet does not mix with another photostimulated luminescent light beam emitted by another excitation light beam irradiated onto the sheet adjacent to the one excitation light beam.
In order to enhance the degree of convergence of the excitation light beam irradiated onto the sheet, it is desirable to dispose a cylindrical lens, a slit, a refractive index profile type lens array, a rod lens array, a fluorescent-light guiding sheet, optical fibers, etc., or a combination of them, between the light source and the sheet.
In the case where a light-emitting element array with a plurality of light-emitting elements disposed in a row is employed as the excitation light source, a portion joining light-emitting elements together or an electrode portion becomes a nonemission portion that emits no light. Therefore, in the case where the light-emitting element array is employed as the horizontal scan means, it is preferable to use a plurality of light-emitting element arrays. In the arrays, the nonemission portions in one light-emitting element array and the light-emitting elements in another light-emitting element array are complementarily disposed in the vertical scanning direction. Also, the light-emitting elements in the light-emitting element arrays are sequentially turned on alternately between the light-emitting element arrays so that light emitted from the light-emitting element arrays is scanned horizontally onto the phosphor sheet as a single excitation light beam.
The expression xe2x80x9ccomplementarily disposedxe2x80x9d means that one light-emitting element array and another light-emitting element array are disposed so that the nonemission portions in the one light-emitting element array and the light-emitting elements in the other light-emitting element array are aligned with one another in the vertical scanning direction and so that the light-emitting elements of the one array are not aligned with those of the other array
In the radiation image information reader of the present invention, the read means may have a line sensor in which a plurality of photoelectric conversion elements are disposed in the horizontal scanning direction. Also, the read means may have an area sensor in which a plurality of photoelectric conversion elements are two-dimensionally disposed.
Furthermore, the read means may comprise light-collecting means having a plurality of separate portions which respectively collect the photostimulated luminescent light beams, and a plurality of photoelectric conversion means for respectively performing photoelectric conversion on the photostimulated luminescent light beams collected by the light-collecting means. When two photostimulated luminescent light beams emitted by irradiation of two excitation light beams are collected by one of the separate portions during horizontal scanning of the plurality of excitation light beams, the horizontal scanning is performed by turning off one of the two excitation light beams so that one of the two photostimulated luminescent light beams is turned off.
The line sensor can employ an amorphous silicon sensor, a charge-coupled device (CCD) sensor, a CCD with a back illuminator, a metal-oxide-semiconductor (MOS) image sensor, etc.
The line sensor may be constructed by disposing a plurality of photoelectric conversion elements in the vertical scanning direction. In this case, an array in which a plurality of photoelectric conversion elements are disposed is not limited to a matrix array in which conversion elements are disposed straight in both the longitudinal direction and the transverse direction. It may be an array in which conversion elements are disposed straight in the longitudinal direction and zigzag in the transverse direction, an array in which conversion elements are disposed straight in the transverse direction and zigzag in the longitudinal direction, or an array in which conversion elements are disposed zigzag in both the longitudinal direction and the transverse direction.
The horizontal scan means and the read means may be disposed on the same side with respect to the sheet, or may be disposed separately on the opposite sides across the sheet. Furthermore, two read means maybe disposed on the opposite sides across the sheet, respectively. In the case where the horizontal scan means and the read means are disposed separately on the opposite sides, or two read means are disposed on the opposite sides, a support body for the sheet needs to have a property of allowing passage of a photostimulated luminescent light beam so that the photostimulated luminescent light beam is transmitted through the bottom surface of the sheet opposite to the sheet surface on which an excitation light beam is incident.
According to the present invention, a plurality of spot-sized excitation light beams are scanned simultaneously onto a stimulable phosphor sheet at predetermined intervals on the horizontal scanning line, so the excitation light beams can be efficiently irradiated onto the sheet. This can shorten the time needed to read the radiation image information stored in the sheet. In addition, since horizontal scanning is performed with the spot-sized excitation light beams, crosstalk does not occur as it does in the case where line excitation light is irradiated onto the sheet. This makes it possible to obtain an image signal with which a high-quality radiation image without a reduction in sharpness due to crosstalk is reproducible.
In addition, in the case where a light-emitting element array with a plurality of light-emitting elements disposed in a row is employed as the excitation light source, a plurality of light-emitting element arrays are used. In this case, the nonemission portions in one light-emitting element array and the light-emitting elements in another light-emitting element array are disposed complementarily in the vertical scanning direction. Furthermore, a single excitation light beam is emitted by sequentially turning on the light-emitting elements of the light-emitting element arrays alternately. This renders it possible to irradiate an excitation light beam smoothly and continuously without being affected by the non emissive portion between the light-emitting elements of the light-emitting element array.
Furthermore, in the case where the read means comprises light-collecting means having a plurality of separate portions which respectively collect the photostimulated luminescent light beams emitted by horizontal scanning of excitation light beams, and a plurality of photoelectric conversion means, two photostimulated luminescent light beams emitted by two excitation light beams will be collected by a single separate portion, if the excitation light beam scans that portion on the sheet that corresponds to a portion near the boundary of the aforementioned separate portions. If the two photostimulated luminescent light beams are detected by one separate portion of the light-collecting means, it will cause crosstalk. Therefore, in the case where two photostimulated luminescent light beams are detected by a single separate portion during horizontal scanning of excitation light beams, horizontal scanning is performed by turning off one of the two excitation light beams so that one of the two photostimulated luminescent light beams is turned off. Thus, the occurrence of crosstalk can be prevented.